Helical antenna construction



m- 1956 E. F. HARRIS 2,763,003

HELICAL ANTENNA CONSTRUCTION Filed July 1, 1953 United States PatentHELICAL ANTENNA CONSTRUCTION Edward F. Harris, Lincolnwood, lll.Application July 1, 1953, Serial No. 365,355

3 Claims. (Cl. 343-895) This invention relates generally to highfrequency antennas and more particularly is concerned with aconstruction of a helical antenna.

The helical antenna, as it is known, is an unusual type of antenna dueto its characteristics which render it distinct from the other type ofhigh frequency antenna such as yagis, paraboloids, and cornerreflectors. The primary consideration is that all other antennas arelinearly polarized, usually vertically, while the helical antenna iscircularly polarized. For unidirectional service, therefore, it isapparent that the most desirable of antennas is the helical, because itwill provide a beam which is not only circularly polarized, butpolarized either right handed or left handed, depending upon the mannerin which the helix winds. Its transmitted wave can be received only byan antenna which is likewise polarized circularly and in the same sense.

Along the propagation path of the usual linearly polarized antenna,interference between the directly received wave and the reflected waveresults in fading, diurnal effects, and the like. Since the reflectedwave from a circularly polarized antenna rotates substantially in theopposite direction, the receiving antenna will only receive the primarywave. It will be substantially free from the fading. andatmospherica-l-ly caused phenomena attendant upon the use of linearlypolarized antennas. Tests have shown that helices of opposite rotationalsense can discriminate side by side, i. e., where one is properlyarranged to receive the transmitted wave and the other is not, by asignal difference of the order of 20 db. This ability to discriminatealso makes possible efficient adjacent and back-to-back systems.

Accordingly, it is a principal object of the invention to provide auni-directional antenna which is constructed to afford all of the abovementioned advantages of the helical beam.

Other considerations of the helical beam antenna are in the matter ofgain, cost, and critical dimensions. At 450 megacycles, for which thisinvention is especially suitable, the three types of linearly polarizedantennas mentioned heretofore afford good gain. The corner being of theorder of 8 to 10 db, the yagi being even better, and the paraboloidexceeding even that. The gain through the use of the helical beamantenna is of the same order as that of the paraboloid.

The cost of installation of the corner reflector and the paraboloid, atleast of such dimensions as to give performance comparable to that of agiven size of helical antenna is considerably greater than the cost ofinstallat-ion of the helical antenna. Even the yagi requires expensivefeed lines, and systems which are anything but simple. All of theseantennas require feeding by means of dipole or modified linearradiators. Transformation from the unbalanced mode of the almostuniversally used coaxial cable and the feed balanced dipole is anexpense and complication which are also eliminated by the constructionof my invention. The helical beamantenna presents substantially anatural termination for the coice '2 axial cable transmission linefeeding the same in the construction described hereinafter. The outerconductor of the cable is connected to the ground plane, and the helixitself is a continuation of the central conductor.

The Wave on the helix is a travelling wave and hence attenuatesappreciably by the time itv reaches the end of the helix. There is verylittle reflected wave, therefore, and the coaxial cable line haspractically constant terminal impedance.

The provision of such advantages of the helical antenna over others areother objects of the invention.

A most important consideration in using antennas of the kind discussedhereinabove is that the dimensions of such antenna are critical, makingthem difficult to manufacture, maintain, and mount, but more importantthe existence of ice built up upon these antenna not only distorts theirpattern but substantially lowers their efficiency. As an example, theyagi, which incidentally has less band width than either of the othertwo linearly polarized antennas and certainly less than the helicalantenna, can lose as much as 50% efficiency through build-up of snow andice. The helical antenna is characterized by a complete lack of criticaldimensions and adjustments. It has wide band width, high operatingefficiency, high radiation resistance, and its pattern and output arepractically unaltered by staggering loads of snow and ice.

The use of helical beam antennas has been heretofore greatly limitedbecause of the difficulties in constructing the same and mounting thesame. cantilever structure of the antenna, proper non-metallic supportcapable of withstanding severe weather conditions has not been devised.It is therefore an important object of the invention to provide astructure which is simple, economical, durable and above all has greatstrength.

A further object of the invention is to provide a helical beam antennawhich is formed of simple and easily obtained parts, to provide anantenna construction through the use of which the antenna is renderedtotally weatherproof, and to provide an antenna which will at the sametime have all of the advantages enumerated above.

Another important object of the invention is to provide a novel methodfor manufacturing the antenna of the invention in a speedy andeconomical manner.

Many other objects will occur to those skilled the art to which theinvention appertains. For enabling a full :and clear understanding ofthe invent-Len, its structure, assembly, use and mode of constructionand practice I have illustrated a preferred embodiment and described thesame in considerable detail from which it will become apparent that Ihave made a substantial and important advance in the arts and sciences.

In the drawings:

Fig. l is a perspective view of an installation of -a helical beamantenna constructed in accordance with my invention.

Fig. 2 is a side elevational view of the same with portions broken awayto show the interior construction thereof.

Fig. 3 is a sectional view on an exaggerated scale to show the manner inwhich the conductor is laid upon the core of the antenna.

Fig. 4 is a sectional view taken generally along the line 44 of Fig. 2and in the direction indicated.

Fig. 5 is a diagrammatic sectional view showing the connection betweenthe coaxial cable and the antenna.

Fig. 6 is a perspective view showing the antenna in Due to the inherent.

The problems attendant upon point to point communications at relativelyhigh frequencies are capable of solution through the use of helicallypolarized antennas, but until the advent of the invention herein the useof such antennas were impractical. This was true principally because ofdifficulties of construction, mounting and support. My antenna has beendeveloped principally in connection with use in the so-called citizensfrequency band, i. e., from 450 to 470 megacycles, but is applicable toother frequencies as well. As a matter of fact, tests which I haveconducted on antennas specifically designed for 450 megacycles show thatthe side lobes and non-uni-directional effects (i. e. spuriousresponses) are substantially negligible between 390 megacycles and 600megacycles.

I have provided a novel method of producing an antenna of uniqueconstruction which will render practical the use of helical beamantennas. As for the article of manufacture, it consists of acylindrical core member of cellular plastic having its axis coincidentwith the line of propagation, having the conductor of flexible braidenwrapped about the surface thereof in helical formation, and the wholebeing sealed in a cylindrical structure of layers of fiberglassimpregnated with resin. The whole is mounted upon a base plate andprovided with suitable supports, the transmission line being connectedto the back of the base plate with its outer conductor electricallyconnected to the base plate and its inner conductor connected directlyto the antenna element. As for the novel method of manufacture, itconsists of mounting the cylindrical core upon the plate and placing theassembly upon a lathe-like rotating mandrel, cementing one end of anelongate sheet of glass fibre cloth upon the surface of the cylinderwith bonding resin, and then slowly rotating the mandrel and hence thecylinder while enwrapping the sheet about the same, the operator in themeantime applying the resin coating with a brush, much like glue. Afterthis, the assembly is cured in a mold. A cap of resin or resinimpregnated fiberglass may be applied to the end either before or afterthe sheet is enwrapped upon the cylinder, and is cured in the mold also.

Referring now to the figures, in Fig. 1 there is illustrated an antennawhich is mounted for uni-directional service upon a pole or tower 11.The direction of propagation or reception of the antenna 10 iscoincidental with the axis of the cylinder comprising the antenna aswill shortly be explained. The antenna 10 comprises a central elongatecylindrical core 12 which has its left end as viewed in the figuresengaged within the upstanding cylindrical ring 13 secured to an annularbase plate 14. The base plate and ring may be integrally formed bymoulding or may merely consist of a shaped angle iron or strap. Theplate 14 may be riveted or welded to a relatively large rectangularmember 22 comprising the antenna ground plane and hereinafter referredto as such. Obviously this ground plane 22 is formed of metal. Thepreferred arrangement need not have the base plate 14, the ring 13welded to the ground plane plate 22 being satisfactory.

Wound about the core 12 is a length of conductor 17 which is preferablyformed of pliable metal braid of the conventional construction usefulfor shielding purposes. This conductor provides a flexible easily workedmaterial which has good current carrying capacity, and which will lieflat. I form the conductor 17 into a plurality of equidistant helicalconvolutions, having a sense which depends upon the system to be used.For example, in the example shown, looking out from the antenna alongthe line of propagation, or reception (since the identical constructionis used in both cases), the conductor 17 is wound to the right orclockwise going out. The antenna is therefore intended to be used with amate which has its conductor similarly wound.

In the particular embodiment illustrated, the antenna 4 was cut for 450megacycles, and there were six turns of braid of 14 pitch, so that theoverall length of the entire structure, including end cap, to the groundplane was approximately 29 inches. The total diameter of the antenna wasabout 6 /2 inches, the ground plane being formed of quarter inchaluminum 16 inches square. The braid was of copper, normally inch indiameter.

The core 12 is formed of a cellular or foam plastic, of extremely lightweight and easily worked. This material has excellent dielectricproperties for use in antenna constructions. This core is approximately6 inches in diameter for the example described, and preferably is a bitmore than that when the outer crust which is molded over the core is inplace, the thickness of the crust is about Ms inch. In the figures thisthickness is exaggerated in order better to illustrate the construction.

The outer crust 19 comprises a multiplicity of layers of fiberglassfabric or cloth, several of which are illustrated at 20,well-impregnated with a liquid thermosetting plastic resin, preferablyof the polyester type, so that the convolutions of the conductor 17 areembedded in the crust and the entire core and ring 13 likewise sealedagainst the atmosphere. An end cap 21 of plastic, either formed oflayers of impregnated fiberglass or of the resin itself is provided.Many methods of moulding the antenna 10 may be utilized, but I prefer anovel process which will hereinafter be described.

The ground plane 22 mounts a coaxial connector 23 which is of the typecommercially known as type N. It enables a coaxial transmission line 24to be led to the antenna and suitably connected thereto. In Fig. 5 thereis illustrated diagrammatically the simple connection between theantenna and the coaxial cable. For convenience the connector is notshown. The coaxial cable includes an outer insulating cover 25 and anouter braided conductor 26 just inside the insulating cover. The centralconductor 27 is embedded in the central insulating layer 28. The centralconductor 27 extends through the ground plane plate 22 and is insulatedtherefrom as indicated by the insulating member 29 and is electricallyconnected at 30 to the inner end of the antenna conductor 17. Note thatthe opposite end of the antenna conductor 17 is not connected toanything but merely poised in free space. There is a bond between theconductor 26 and the ground plane plate 22 as indicated by theconnection 31.

The plate 22 has swivel brackets 34 bolted thereto by means of which theentire assembled antenna may be secured to the pole 11, as for exampleby shackles, tie-rods, bolts, etc. Obvious means of securement willsuggest themselves to the artisan. It is only of importance that sincethe antenna is provided with a simple and rugged ground plane plate 22which serves as mounting means for the physical antenna, considerablesavings in effort and expense is effected in installing the antenna.

In manufacturing the antenna, I have devised a novel and effectivemethod which may be practiced by workmen not possessing a high degree ofskill, and yet the resulting product is of excellence which would beachieved through the use of more expensive methods, by diemoulding, forexample. First, the ring 13 is welded to the ground plate 22. This ringmay or may not have the base plate flange 14. Next, the connector 23 ismounted on the plate 22 and a proper length of braid 17 is measured outand soldered to that portion of the connector 23 which extends throughthe plate. For example, as shown in Fig. 5, the braid end is soldered asat 30 to the conductor 27. In the 450 megacycle antenna which I havedescribed, the braid will be approximately 9 feet long. Thereafter, thecore 12 is forced into the ring 13 and the helix is wound on the core.The size of the ring 13 is preferably chosen so that there is a forcefit of the core within the ring.

The helix may be cemented in place or held upon the Surface of thecylindrical core 12 by a pressure sensitive tape, or it may be mountedin grooves formed in the core, the latter expedient not being necessary,but of possible convenience.

As thus far described, the method is fairly obvious from the structureof the antenna, but at this point the thus far described assembly istreated in a novel manner. The plate 22 is bolted to a mandrel 40mounted on a rotatable head 41. This could be for example the head endof a lathe. By the use of a wheel 42 or power, the mandrel 40 is rotatedthereby rotating the core 12. A length of fabric of glass fibres is thenstarted upon the core. The length is illustrated at 43 and it is out sothat when completely enwrapped about the core, the desired finaldiameter will be attained. The leading end 44 is laid down and cementedin place by the use of the plastic resin that is used for bonding andimpregnating the fabric. The mandrel 40 is slowly turned while thelength of fabric 43 is laid upon the core, enwrapping the same. As thisprocess proceeds, the resin in fluid form is literally painted upon thecore and the fabric with a brush, this being illustrated generally at45. The resin is liberally applied and the fabric tightly wrapped sothat the resin will ooze through the interstices of the fabric andcompletely impregnate the same. When the crust is cured, the result issubstantially an integral wall.

Simultaneously with applying the layers 20, the end cap 21 may beapplied, or if desired, this can be done in the next step. The antennais removed'from the mandrel 40, and a mold such as the hinged device 47is engaged upon the cylindrical portion. The halves of the mold aretightened and the mold permitted to remain in this condition until allof the resin has hardened. The end cap may merely consist of a plugformed by filling the upper end of the mold with fluid resin once theantenna has been placed therein. A flat plate 48 engaged upon the endand pressed upon the cap will provide a smooth end surface.

It will be noted from the figures that the ring 13 is moulded into theinterior of the antenna so that any danger of leaking at this point iseliminated. This is done simply by having the fabric layers cover thesame so that the crust formed after curing of the resin engages over thering.

It is believed that the invention has been sufficiently described suchas to enable those skilled in the art to 6 which the same appertains toconstruct and use the same. Many variations are possible without in anyway departing from the spirit or scope Otf the invention as defined inthe appended claims.

What is claimed by Letters Patent of the United States is:

1. A helical, circularly polarized antenna horizontally arranged andcantilever mounted with no metallic members along its electrical length,said antenna comprising a vertically planar metallic base plate andmeans for securing said base plate to an upright, a circular flangemember having a central cavity secured to said base plate with its axishorizontal, an elongate cylindrical core member of cellular plastichaving one end in said cavity and being coaxial with said flange memberwhereby to protrude perpendicularly from said base plate with itsopposite end unsupported, a flexible conductor helically engaged aboutthe core member surface along the length thereof in configuration toprovide helically polarized characteristics for said antenna along theaxis of said core member, said core member, flexible conductor andflange member having a thin coating of thermosetting resinous plasticadhesively securing and bonding said core member and coating to saidflange member and base plate rendering said antenna impervious tomoisture While providing substantial cantilever support therefor, and atransmission line having one lead insulatedly extending through saidbase plate and electrically connected with the inner end of saidflexible conductor and having a second lead electrically engaged withsaid base plate.

2. An antenna as described in claim 1 in which the flexible conductorcomprises flattened metal braid.

3. An antenna as described in claim 1 in which said thin coatingconsists of glass fibres impregnated with a polyester resin.

References Cited in the file of this patent UNITED STATES PATENTS2,296,781 Farny Sept. 22, 1942 2,552,599 Stout May 15, 1951 2,575,377Wohl Nov. 20, 1951 2,590,493 Berberich et al. Mar. 25, 1952 2,614,058Francis Oct. 14, 1952 2,630,530 Adcock et a1 Mar. 3, 1953 2,637,533Sheldorf May 5, 1953

